They already had Singer, in a Producing role. He wrote the story for First Class, and was planning on directing.

And Vaughn only directed First Class because Singer couldn't.

Why do you think Vaughn dropped out of DOFP? (besides his normal tendency to...)

I'll give you a hint:

The studio had a change of thought midway through development, due to various factors.
And then...'creative differences' reared their ugly head.

In many peoples' opinions, Singer's back where he should be (and where FOX originally wanted him to be), at the helm.

We're agreed on everything else.

Vaughn has never been reliable on projects, he jumped ship for secret service because he was told if he wanted to do secret service he had to start straight away, that's the reason why he ain't directing it

You can tell Vaughn wanted to make a James Bond style film that's obvious just by looking at FC and some of his comments and he will probably bring some of that to secret service